The subject invention is directed generally toward the metal working art and, more particularly, to a method of forming holes or recesses to close tolerances in high hardness steel.
The invention is especially suited for use in manufacturing percussion drilling bits of the types used in rock drilling and mining operations and will be described with particular reference thereto; however, the invention is capable of broader application and could be used to produce roller cone bits, polycrystalline diamond compact (PDC) bits, or similar bits of the type wherein the cutting is performed by hard material inserts carried in a steel body or whenever it is desired to produce a high hardness steel body having closely toleranced holes or recesses formed therein.
One type of bit often used in rock drilling operations comprises a steel drill body having a generally cylindrical shank. The axial end face of the body is provided with a multiplicity of cylindrical openings into which are press-fitted cylindrical inserts of sintered tungsten carbide or the like. The outer ends of the carbide inserts have a generally hemispherical shape and extend outwardly of the end face of the steel body. The inserts perform the actual cutting or drilling of rock through an abrading or localized crushing operation.
As drilling takes place, the inserts are slowly worn away and bit life is generally dictated by the life of the inserts. In certain rock formations, however, the nature of the rock is such that the rock dust and particles generated during the drilling operation erode the face of the steel bit body. This erosion of the body can proceed significantly faster than the wear on the inserts and bit failure takes place not because of insert wear but due to loss of support for the inserts. This wear and loss of support is generally most severe with respect to the outer or "gage row" inserts.
In an effort to overcome the noted problem, various attempts have been made at increasing the hardness of the steel body. However, such attempts have not been commercially successful. For example, heat treating the body to high hardness prior to forming the insert openings makes forming the openings an extremely time consuming operation since conventional drilling, or drilling and boring operations cannot be used. Rather, slower and more expensive processes such as electrical discharge machining and/or grinding must be employed.
Similarly, attempts at forming the insert openings while the steel body is in a soft or fully annealed state and thereafter heat treating the body to high hardness have been unsuccessful. The reason for this has been that the holes do not retain their required tolerances following the heat treating. That is, after heat treating there are found to be relatively wide and varying differences in the diameters of holes which prior to heat treating were of uniform diameter. As a consequence, the carbide inserts cannot be properly press-fitted in the openings.
As is apparent from the foregoing, a distinct need exists for a method of forming the required openings to close tolerances while also providing a drill body of high hardness.